Streamed JSONL responses with no content-type header are returned unparsed

[jimryan]

Bug: messages.batches.results_streaming yields raw JSON strings instead of coerced model objects when the results response has no content-type header

Environment

• anthropic gem: 1.49.0
• Ruby: 4.0.3

Summary

Iterating a batch results stream yields raw JSONL line Strings instead of coerced Anthropic::Models::Messages::MessageBatchIndividualResponse objects.
Any attribute access on the yielded value raises NoMethodError.

client = Anthropic::Client.new

client.messages.batches.results_streaming("msgbatch_...").each do |result|
  result.custom_id
  # => NoMethodError: undefined method 'custom_id' for an instance of String
end

The yielded objects are the raw JSONL lines:

item = nil
client.messages.batches.results_streaming("msgbatch_...").each { |r| item = r; break }

item.class    # => String  (expected: Anthropic::Models::Messages::MessageBatchIndividualResponse)
item[0, 60]   # => "{\"custom_id\":\"foo-1\",\"result\":{\"type\":\"succeeded\",\"mes"

Expected

The stream should yield coerced MessageBatchIndividualResponse objects (.custom_id, .result.type, .result.message, etc.), as the type signature and docs imply.

Root cause

The batch-results endpoint returns the .jsonl body with no content-type response header:

require "net/http"
uri = URI("https://api.anthropic.com/v1/messages/batches/msgbatch_.../results")                                                                   
req = Net::HTTP::Get.new(uri)
req["x-api-key"] = ENV["ANTHROPIC_API_KEY"]
req["anthropic-version"] = "2023-06-01"
req["accept"] = "application/x-jsonl"
res = Net::HTTP.start(uri.host, uri.port, use_ssl: true) { _1.request(req) }

res.code              # => "200"
res["content-type"]   # => nil

Anthropic::Internal::Util.decode_content (lib/anthropic/internal/util.rb) decides whether to parse JSONL by matching the response content-type against a regex:

anthropic-sdk-ruby/lib/anthropic/internal/util.rb

Lines 743 to 771 in 67f5d27

	def decode_content(headers, stream:, suppress_error: false)
	case (content_type = headers["content-type"])
	in Anthropic::Internal::Util::JSON_CONTENT
	return nil if (json = stream.to_a.join).empty?
	begin
	JSON.parse(json, symbolize_names: true)
	rescue JSON::ParserError => e
	raise e unless suppress_error
	json
	end
	in Anthropic::Internal::Util::JSONL_CONTENT
	lines = decode_lines(stream)
	chain_fused(lines) do |y|
	lines.each do
	next if _1.empty?
	y << JSON.parse(_1, symbolize_names: true)
	end
	end
	in %r{^text/event-stream}
	lines = decode_lines(stream)
	decode_sse(lines)
	else
	text = stream.to_a.join
	force_charset!(content_type, text: text)
	StringIO.new(text)
	end
	end

Because content_type is nil, none of the case/in regex patterns match (Regexp === nil is always false), so it falls into the else branch and returns the unparsed body as a StringIO.

JsonLStream#iterator (lib/anthropic/internal/json_l_stream.rb) then iterates that StringIO line by line and coerces each:

anthropic-sdk-ruby/lib/anthropic/internal/jsonl_stream.rb

Lines 18 to 22 in 67f5d27

	@iterator ||= Anthropic::Internal::Util.chain_fused(@stream) do |y|
	@stream.each do
	y << Anthropic::Internal::Type::Converter.coerce(@model, _1)
	end
	end

Converter.coerce(MessageBatchIndividualResponse, "<raw json string>") can't convert a String to the model, so it leniently returns the string unchanged — and raw strings reach the caller.

So the request side already knows the format — Resources::Messages::Batches#results_streaming sets accept: application/x-jsonl and wraps the response in JsonLStream — but the parse decision in decode_content keys off the (absent) response content-type instead of the expected type.

Suggested fixes (any one resolves it)

1. In decode_content, when the response is being consumed as a JsonLStream (or when accept/expected type is JSONL), force the JSONL decode path rather
   than relying solely on the response content-type header.
2. Make JsonLStream#iterator JSON-parse raw String lines itself (defensive against an upstream that yields unparsed lines).
3. Have results_streaming pass its known expected content type into the decode step so a missing response header falls back to JSONL rather than the raw else branch.

Workaround

Parse + coerce each line manually (note: coercion requires symbol keys):

MODEL = Anthropic::Models::Messages::MessageBatchIndividualResponse

client.messages.batches.results_streaming(batch_id).each do |raw|
  result = raw.is_a?(String) ? Anthropic::Internal::Type::Converter.coerce(MODEL, JSON.parse(raw, symbolize_names: true)) : raw
  # result.custom_id, result.result.type, result.result.message, ...
end

[felixfbecker]

Hi @jimryan, I am unable to reproduce the lack of a Content-Type header on the batch API response. Do you have a request ID by any chance? It's returned in the anthropic-request-id response header.

[jimryan]

@felixfbecker Thanks for the lightning quick reply!

I do not see an anthropic-request-id header, just a request-id header, and it's req_011CcMX5fp1Qk18CccicUGSr

Here's how the SDK handles that same request:

>> client.messages.batches.results_streaming("msgbatch_...").map(&:class)
=> [String, String, String, String, String, String]

[felixfbecker]

Thanks! and to confirm, that request ID did not send Content-Type? Could you post a dump of all the headers?

[jimryan]

That is correct

❯ curl -I --request GET \
  --url https://api.anthropic.com/v1/messages/batches/msgbatch_<REDACTED>/results \
  --header 'accept: application/x-jsonl' \
  --header 'anthropic-version: 2023-06-01' \
  --header 'x-api-key: <SECRET>' 
HTTP/2 200 
date: Thu, 25 Jun 2026 18:36:21 GMT
content-disposition: attachment; filename*=utf-8''msgbatch_<REDACTED>_results.jsonl
request-id: req_011CcQTjrRqS7EuqjaNNM4Ru
strict-transport-security: max-age=31536000; includeSubDomains; preload
anthropic-organization-id: <REDACTED>
server: cloudflare
x-robots-tag: none
vary: Accept-Encoding
server-timing: x-originResponse;dur=872
cf-cache-status: DYNAMIC
set-cookie: _cfuvid=<REDACTED>; HttpOnly; SameSite=None; Secure; Path=/; Domain=api.anthropic.com
content-security-policy: default-src 'none'; frame-ancestors 'none'
cf-ray: a11613c7ab3309ae-EWR